The present invention relates to a method of monitoring a vascular access during an extracorporeal blood treatment, in particular a chronic blood purification therapy such as hemodialysis, hemofiltration and hemodiafiltration and an apparatus for extracorporeal blood treatment, in particular for hemodialysis, hemofiltration and hemodiafiltration with a device for monitoring the vascular access.
With the known methods of chronic blood purification therapy such as hemodialysis, hemofiltration and hemodiafiltration, the patient""s blood is passed through an extracorporeal system. Arteriovenous fistulas, vascular implants and/or various catheters are used as access to the patient""s vascular system. Typical flow rates in a vascular access are in the range of 1100 mL/min. The patient is usually connected to the extracorporeal system by dialysis cannulas which are used to tap into the fistula or the vascular implant.
If the connection between the extracorporeal system and the vascular system becomes undone or a blood leak occurs in the extracorporeal system, the patient can be prevented from bleeding to death only if the extracorporeal blood flow is stopped within a few minutes. Therefore, extracorporeal blood systems are generally equipped with protective systems to constantly monitor the arterial and venous pressure (Part and Pven) within the system as well as the admission of air into the extracorporeal system.
In the event of an alarm, the blood treatment is stopped, the venous clamp is closed and acoustic and optical warning signals are triggered. The protective system based on the pressure measurement responds when the arterial or venous pressure in the extracorporeal system changes by more than xc2x160 mm Hg. The alarm limits are selected so that a change in the position of the patient does not trigger an alarm.
If the connection between the patient and machine becomes undone at the arterial connection, i.e., at the cannula establishing the patient""s blood flow to the extracorporeal system, the pressure-based protective system on the machine responds rapidly, as explained below. The dialysis cannula presents the greatest flow resistance in the extracorporeal system. When air is drawn through the cannula into the arterial vacuum system of the extracorporeal system, the flow resistance of the cannula drops by a factor of 103 in proportion to the difference in density between the blood and air. Thus, the arterial vacuum in the extracorporeal system collapses suddenly.
However, the response of the pressure-based protective system is not always guaranteed in the case when the venous cannula has become detached from the vascular access. On the venous side, the purified blood is supplied to the patient at an excess pressure, with the venous excess pressure being proportional to the delivery rate of the blood pump. This prevents penetration of air through the cannula into the extracorporeal system, which would be the case on the arterial vacuum side. Therefore, the flow resistance of the venous cannula does not change, and the venous pressure on the machine end drops only by the amount of the pressure in the patient""s vascular access. Thus, the change in venous pressure in the extracorporeal system is usually too low to trigger the pressure-based protective system to respond. The additional hydrostatic pressure difference between the venous pressure sensor and the cannula triggers a machine alarm only when the venous cannula is definitely below the fistula after slipping out of the vascular access.
In the case of a blood leak in the venous tubing system, it may also occur that the resulting venous pressure drop is not sufficient to guarantee that the existing pressure-based protective system will be triggered.
In addition to the above method, where the pressure in the arterial branch of the extracorporeal system is monitored to detect whether the arterial cannula has slipped out, and where the pressure in the venous branch of the extracorporeal system is monitored independently of the pressure monitoring in the arterial branch to detect whether the venous cannula has slipped out, there are known monitoring systems that monitor pressure pulses propagating in the extracorporeal system.
International Patent No. WO 97/10013 describes a dialysis machine having such a monitoring system which monitors the pressure pulses in the venous blood line produced by the blood pump in the arterial blood line.
An object of the present invention is to provide a method of monitoring a vascular access during an extracorporeal blood treatment that allows reliable detection of the venous cannula slipping out of the vascular access as well as reliable detection of a blood leak in the venous branch of the extracorporeal system while also requiring a minimum of equipment. This object is achieved by a method of calculating certain characteristic values from the pressure in a patient""s extracorporeal blood path during treatment and comparing these characteristic values to predetermined ranges for these values. The method includes monitoring a vascular access during an extracorporeal blood treatment, wherein a patient""s blood is provided from the vascular access to an extracorporeal blood path. The extracorporeal blood path includes an arterial line which is in fluid communication with an arterial access at one end and the inlet of a blood treatment device at the other end. The blood path also includes a venous line extending from and in fluid communication with an outlet in the blood treatment device and in fluid communication with a venous vascular access. According to the method, the fluid pressure within the arterial and venous lines is measured and from these measured values characteristic values for the integrity of the vascular access are calculated. The integrity of the vascular access is determined by comparing the characteristic values to ranges of predetermined values.
Another object of the present invention is to create an apparatus for extracorporeal blood treatment with a device for monitoring a vascular access that detects whether the venous cannula has slipped out of the vascular access as well as whether there is any blood leak in the venous branch of the extracorporeal system with a high reliability while requiring a minimum of additional equipment. This object is achieved by a system that will calculate certain characteristic values from the pressure in a patient""s extracorporeal blood path during treatment and compare the calculated characteristic values to predetermined ranges for these values. The system according to the invention includes an extracorporeal blood path having an arterial line which is in fluid connection with an arterial vascular access and an inlet to a blood treatment device. The system further includes a venous line which is in fluid connection with an outlet of the blood treatment device and a venous vascular access. The system also includes an arterial fluid pressure sensor connected to the arterial line that is adapted for measuring pressure within the arterial line and a venous fluid pressure sensor connected to the venous line that is adapted for measuring pressure within the venous line. The system further includes a computer unit connected to the arterial and venous pressure sensors, which is adapted for generating characteristic values related to the integrity of the vascular access from measured arterial and venous pressures. Also provided in the system according to the invention is an analyzer unit that is adapted for comparing the calculated characteristic values to predetermined ranges values in order to assess the integrity of the vascular access.
The method according to the present invention can be designed as a protective system integrated into the machine, using sensors that are already present in the known blood treatment machines. Thus, the only required change in the machine to implement the protective system is modifying the machine control.
The method according to the present invention is based on the fact that both the pressure in the arterial branch and the pressure in the venous branch of the extracorporeal system are monitored to detect when the venous cannula slips out of the vascular access or when there is a blood leak in the venous branch of the extracorporeal system. Values characteristic of the condition of the vascular access are calculated from the pressure in the arterial and venous branches of the extracorporeal system and are then analyzed to detect a defective vascular access.
With the method according to the present invention, it is possible to reliably detect not only whether the venous cannula has slipped out or whether there is a blood leak in the venous branch of the extracorporeal system, but also whether the arterial cannula has slipped out and whether there is a blood leak in the arterial branch of the extracorporeal system.
The method according to the present invention can also be combined with other methods of detecting a defective vascular access. This further increases the reliability of the monitoring system.
For the case when the vascular access is defective, an acoustic and/or a visual alarm is preferably triggered. In addition, the blood flow in the extracorporeal system can be interrupted to prevent any blood loss. The blood flow can be interrupted using a suitable apparatus for extracorporeal blood treatment, such as by stopping the blood pump arranged in the extracorporeal system and/or closing a safety valve, such as a hose clamp arranged in the extracorporeal system.
Monitoring for a defective vascular access with the method according to the present invention is possible not only in an apparatus for hemodialysis, hemofiltration or hemodiafiltration, but also in cell separators in which a donor""s blood is centrifuged in an extracorporeal system, thereby separating it into its components.
The method according to the present invention for monitoring a vascular access and an apparatus for extracorporeal blood treatment with a device for monitoring the vascular access are explained in greater detail below with respect to the drawings and on the basis of a preferred embodiment.